It has been known to use a phototransistor and a photo-conductive cell for ambient light detection in combination with a photovoltaic device (herein sometimes called a solar panel) to charge a battery for illuminating a sign at night, as shown in U.S. Pat. No. 4,009,535 of Stock. U.S. Pat. Nos. 4,314,198 of Rogers and 4,384,317 of Stackpole note that a timer may be used in lieu of a photoresistor for turning on an illuminated sign at night where the sign is powered by a solar-powered rechargeable battery.
From U.S. Pat. No. 4,486,820 of Baba et al it is known to use the state of a current-producing solar cell as a way of detecting the level of ambient light and, during darkness, to use a timer to disconnect a rechargeable battery before it drains.
U.S. Pat. No. 3,317,809 of Bowers et al discloses two solar panels, one for using ambient light to power an electro-mechanical switch, the other panel being used to charge a battery. U.S. Pat. No. 4,009,051 of Kazis et al also uses mechanical or electromechanical switches, in particular in the case of a solar power pack for use at remote locations.
Some solar powered apparatus, typically having a high number of circuit elements, provide hysteresis or a timer in one or more circuit operations in order to avoid undesired frequent cycling. Examples are given in U.S. Pat. No. 4,333,136 of Baker and U.S. Pat. No. 4,587,459 of Blake.
Other examples of apparatus having rechargeable batteries may be seen in U.S. Pat. Nos. 4,283,657 of Gordon et al, 4,484,104 of O'Brien and 4,661,758 of Whittaker.
The patents listed above represent a range of complexity and therefore a range of cost. In some applications, perhaps represented by the patent of Stock, it is more desirable than in other applications to minimize the number and cost of circuit elements so as to provide the relevant market with low cost products. It is also desirable to eliminate moving parts, as in an electro-mechanical switch. Even so, there is a need to achieve these goal without sacrificing performance, and preferably by enhancing performance.